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5V and 3V together

Hello!

I'm building a board that needs both 3v and 5v as it has some components working on 3v and others on 5v. Now what is the best to do in this case? I am not sure, i've analyzed some of the options but i really do not know the advantages/disadvantages of each:

- have a 5V regulator and a 3V regulator with the input connected to the 5v reg's output (could be done, the 3v reg is low dropout)

- have a 5V reg and some diodes to output 3V (or a voltage divider with resistors)

- have a 5v reg and a 3v reg with both inputs connected to the v+ on the battery 

...i really do not know which is better, could you give me a hand? 

Thank you :=)

ps: i've checked OddBot's super walkthrough already but it does not talk about this.

 

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OddBot's picture

Because you are powering some analog circuitry (DAC) then you want as smooth and as noise free power supply as possible with a constant voltage so using something like 3 diodes in series to drop 5V down to 3V is definitely out. As the current load is small then yes you could run the 3V regulator off of the 5V without any trouble as long as you have at least 100uF capacitance between the 5V output and Gnd and the same again on the output of the 3V regulator. Depending on what load the 5V regulator is driving a 470uF may be a better choice for the 5V regulators output. There should also be a capacitor on the input of the 5V regulator the same size or bigger than the capacitor on it's output.

CaptainTuna's picture

I have 3 220uF capacitors in my setup. The first connects the unregulated V (to ground obviously), the second connects the 5v regulated voltage, and the last connects the 3v voltage. I also added some .1 uF for noise suppression at the same spots. After reading your post sounds like i'm ok... in case i'll have problems in the near future i'll get a higher valued cap on the unregulated voltage.

RobotFreak's picture

I would choose the first option. 3V regulator behind the 5V regulator. For linear voltage regulators the lowest input voltage should be used (less power losses in the regulator). Diodes are ok, if you don't have a voltage regulator ready. Never use a voltage divider with resistors as a power supply!

djhesit8's picture

just use two battery pack and separatly the two regulators with ground connected together, I think this is the best option

CaptainTuna's picture

the problem is that i just have a single battery pack :=(

djhesit8's picture

than as robotfreak said

 

battery ---->   5V regulator  ----> 3V regulator

                            |                         |

                            |                         |

                        out 5V                out 3V

octospider's picture

I don't completely agree to the above setup.

The lower power loss in the 3V regulator will only be added to the 5V regulator loss. You don't get anything for free.

So depending on the load on the 5V and 3V in can actually be better to connect the 3V regulator directly to the battery. The 3V (and 5V) also need a minimum voltage across the input and output to regulate the output voltage stable (LV type might work OK with only 2V accross it)

 

RobotFreak's picture

Yes, that's true. I must correct my answer. It'll depend from the load on the 3V what is better. 5V and 3V rregulator in serie or parallel to the battery.

CaptainTuna's picture

The 3v reg. is currently powering 2 small ICs (a buffer, a DAC) and an SD card, that's all. I guess the load is quite small...so does that mean the above setup is ok?

RobotFreak's picture

The setup above will be ok for this small load.

But you should think about level converter for the controller outputs. Controller inputs can handle 3V as HIGH level normally without any probs. 3V devices normally don't like 5V on input pins if they don't have 5V tolerant inputs.